Common technologies for temperature sensors
Temperature is one of the most frequently measured physical phenomena. Several sensor technologies are available to allow its measurement. Any material with a temperature sensitive characteristic can be used as the basis for a thermometer.
The most common technologies for temperature sensors are:
- Thermocouples,
- Resistive temperature detectors (RTDs),
- Thermistors,
- Several semiconductor devices.
While Thermistors and RTDs are dependent on Resistance versus Temperature, Thermocouples are dependent on Voltage versus Temperature, and Semiconductor Sensor Devices can be Voltage or Current dependent versus Temperature.
Other technologies available for temperature measurement, which deserves a special mention, are the following (although not used for regular applications at home or in buildings):
- Ultrasonic thin-wire thermometer: based on the temperature dependency of the velocity of the sound. This sensor can be applied to monitor high temperatures from 2000 to 3000ºC, like in a nuclear reactor, with a maximum error of 30ºC;
- Quartz-crystal thermometer: based on the fact that the resonant frequency of a quartz-crystal-based oscillator is linearly related to temperature. This sensor can deliver accuracies of 0,04ºC and a range from -80 to 250ºC;
- Johnson noise thermometer: based on the measurement of the noise generated across an unloaded resistor due to thermal activity.
Before going into details for each of the most used technologies, some considerations about scales and associated conversion will be produced.
Temperature scales
There is more than one scale for measuring temperature. The most common scales are: Celsius, Fahrenheit, and Kelvin. It has to be noted that at 1948, Centrigrade scale was replaced by Celsius scale. After 1990, the unit of temperature in the Absolute Temperature or Kelvin scale is the Kelvin.
The invention of the thermometer is attributed to Galileo and the first sealed thermometer was produced by Daniel Gabriel Fahrenheit in 1714. The development of the original Centrigade scale was accomplished in 1742 by Anders Celsius.
The figure below summarizes the temperatures obtained for key-points of the water for the three major scales (Celsius, Fahrenheit and Kelvin), where it is clear the centigrade nature of the Celsius scale (from where the Kelvin scale keeps the difference of 100, although having as absolute zero the temperature of -273.15ºC/-459.67ºF; absolute zero, in physics, means the temperature at which molecular movement virtually ceases and the lowest level of energy is reached).
Water points.
Although having different scales, as all of them are linear, it is easy to convert between them. The following expressions allow us to convert temperatures in different scales.
- From Celsius to Fahrenheit:
F = (9C/5) + 32
- From Celsius to Kelvin
K = C + 273.15
- From Fahrenheit to Celsius
C = 5(F-32)/9